This invention relates to an apparatus for applying multi-component coating compositions and to a process for applying such a coating composition to finish a substrate.
In finish applications, the coating material applied to a substrate as a finish typically is the product of a multi-component coating composition. Generally, in a two-component coating composition, the first component essentially comprises the binder. The second component comprises the hardener required to react with the binder in the first component. The proper mixing ratio of the components is determined by the proper stoechiometric ratios of the reactive parts of the components needed for the reaction to take place. Either component may also contain catalysts for promoting and initiating the reaction, as well as additives, reducers, and pot life extenders. In some cases, more than two components may be involved. For example, a catalyst may be present in a third component.
Presently there is a wide range of apparatus for mixing multi-component coating compositions and delivering them to atomizers or dispensing equipment in proper ratios. For example, such systems are disclosed in U.S. Pat. No. 5,425,968, DE-A-29 14 684, EP-A-0 311 792, EP-A-0 478 944, and GB 1 313 182. However, the dosing cylinders used in these apparatus provide a fixed mixing ratio. If this mixing ratio has to be changed, the apparatus has to be dismantled. If the dosing cylinders are mounted on a rod assembly, they have to be loosened manually and moved along the rod assembly. In other cases, the dosing cylinders have to be changed. It is also possible to mechanically adjust the dosing volume of the cylinders. However, mechanically changing mixing ratios is time-consuming and cannot be repeated exactly.
Also apparatus are known where the mixing ratio is monitored and/or changed electronically. U.S. Pat. No. 4,019,653 discloses such an apparatus where a control unit receives a preset signal representative of the desired flow ratio, monitors the flow rate signals from the respective lines, calculates the actual ratio, compares it with the desired ratio, and adjusts a controllable valve in one connecting line to affect the flow rate and thereby achieve the desired ratio. The disadvantage of this apparatus is that the start-up is not instantaneous. More particularly, for a predetermined time after the spray gun trigger is actuated, the flow rate information is disconnected from the control valve because the initial transient flow signals may be erratic, slow, and unreliable. EP-A-0 300 902 discloses another apparatus comprising a manually controlled pneumatic sprayer operated by a trigger. The compressed air flow rate is varied according to the position of the trigger. The flow rate of the two components is regulated by the signal given by the air flow information. Finally, EP-A-0 644 025 discloses an apparatus wherein the flow ratio of the components is regulated by the pressurized air flow which powers the motor.
One particular application for the above-mentioned apparatus is in the refinish industry. Conventional refinish methods for applying a coating composition to a substrate have been limited in several significant aspects. Typically, the components of the coating composition are mixed manually. Once mixed, the composition must be used within a certain time span, i.e. during its pot life, which is defined as the time when the mixture is suitable for spraying. More specifically, the pot life is the point at which the operator can perceive a discernible difference in ease of handling due to an increase in the viscosity of the mixed components. In general, the pot life is defined as a doubling of the viscosity as measured in a DIN flow cup number 4 according to DIN 53211-1987.
Manual mixing of the components often results in a large amount of waste. More particularly, it is impossible to prepare exactly the same coating composition twice. Accordingly, a large amount of coating composition is prepared before spraying to ascertain that there will be enough coating composition to spray the substrate. Thus, a certain amount of coating composition will always be left over after spraying which cannot be used anymore because of its pot life running out. Hence, the remaining coating composition will be thrown away as chemical waste.
In view of the above, there is need for an improved apparatus for applying a multi-component coating composition.
The present invention is directed to an apparatus for applying multi-component coating compositions comprising at least two dosing devices, an air-assisted spray gun (19), and a controlling device, each dosing device comprising a supply container (3,14) comprising one component of the multi-component coating composition (A,B), a motoring device (11A,11B), and a metering device (9,13), said supply container being connected to said metering device (9,13), said metering device (9,13) being connected to said motoring device (11A,11B) and to said air-assisted spray gun (19), the connecting line (41) between at least one metering device (9) and said air-assisted spray gun (19) being fitted with a pressure transducer (5A) having means for measuring a decrease in pressure in said connecting line (41), said pressure transducer (5A) being connected to said controlling device, and said controlling device being connected to said motoring devices (11 A,11B), said controlling device having means for comparing the decrease in pressure in said connecting line (41) with a set value, means for starting up said motoring devices (11A,11B), and means to control the power of said motoring devices (11A,11B) to keep the pressure in said connecting line (41) to the set value.